Within the Steam Events & Announcements tools, there are three levels of category for game updates: Patch Notes, Regular Updates, and Major Updates.
Patch Notes are important for letting your most engaged players know what has changed in your game, even if those changes are not something most players would care about. Every time you push out a new build to Steam, your players will see their game update in Steam. Many of those players want to see what has changed in the game, so posting something about your update will at least tell players something about the magnitude of the update.
Patch Notes Visibility
Patch Notes are specifically designed to show up just in the appropriate locations. They won't appear at the top of the Library homepage in the "What's New" section.
Instead, these will be accessible when a player clicks to view updates from the Steam client's download window. They will also show up in a compact format on your game's News Hub and within your game's library detail window. The most recent patch notes will also be linked from your store page.
Posting Patch Notes
To post your patch notes, visit your app's landing page in Steamworks and scroll down to "Post/Manage Events & Announcements". Clicking that option will take you to a screen to select the type of event you wish to post. You can click on "A Game Update" to expand the options and then select "Small Update / Patch Notes".
Patch notes are designed to be lightweight and easy to post. Therefore, you really only need to fill out two fields: title and description.
You can easily paste in your bullet list of notes, switch to the 'publish' tab and hit publish.
On top of that, there are various optional settings if you want to provide more information or include images of things that have changed, but are not needed in order to post your patch notes.
Steam-Pipe Builds and Patch Notes
When submitting a new build for your game via SteamPipes, a small prompt will automatically appear to encourage you to also post a patch note for your customers. This prompt lets you easily submit your patch notes inlines at the point after submitting the build. Often developers will use this to write a bullet point list of the changes made. Patch notes following your build updates is helpful for your customers to realize you are continuing work on and improve your game.
You can always use the regular event editor to post the patch notes at a later time. Learning more about this prompting feature here